1. Field of the Invention
The invention relates to a vibration damping control device of a vehicle, such as an automobile. More specifically, the invention is concerned with a vibration damping control device that suppresses vibration of a vehicle body by controlling drive output (driving force or driving torque) of the vehicle having an engine as a drive unit.
2. Description of Related Art
Vibrations, such as pitch and bounce vibrations, during traveling of the vehicle are generated due to braking or driving force (or inertial force) that acts on the vehicle body during acceleration or deceleration of the vehicle, or other external force that acts on the vehicle body. At this time, these forces are reflected by “wheel torque” (torque applied between wheels and a road surface with which the wheels contact) which is applied by the wheels (drive wheels when the vehicle is driven) to the road surface. Thus, in the field of vehicular vibration damping control, it has been proposed to adjust the wheel torque through drive output control of an engine or other drive unit of the vehicle, so as to suppress vibration of the vehicle body during traveling of the vehicle (see, for example, Japanese Patent Application Publication No. 2004-168148 (JP 2004-168148 A), Japanese Patent Application Publication No. 2006-69472 (JP 2006-69472 A), Japanese Patent Application Publication No. 2008-105471 (JP 2008-105471 A), Japanese Patent Application Publication No. 2008-105472 (JP 2008-105472 A), and Japanese Patent Application Publication No. 2009-40163 (JP 2009-40163 A)). In vibration damping control using drive output control as described above, pitch/bounce vibration that arises in the vehicle body when a vehicle acceleration/deceleration request is made or when external force (disturbance) acts on the vehicle body and causes fluctuations in the wheel torque is predicted, using a motion model constructed based on a dynamic model of so-called sprung-mass vibration or sprung-mass and unsprung-mass vibration of the vehicle body, and the drive output of the drive unit of the vehicle is adjusted so as to suppress the predicted vibration. The vibration damping control of this type is advantageous in that the vibration damping operation is relatively quickly performed, and the energy efficiency is good. This is because generation of vibrational energy can be curbed or suppressed by adjusting the source of force that generates vibration, rather than suppressing generated vibrational energy by absorbing the same through vibration damping control using suspensions, for example. Also, in the vibration damping control as described above, an object to be controlled is limited to wheel torque or braking or driving force of the wheel; therefore, the control can be relatively easily adjusted.
In the meantime, during turning of the vehicle, cornering force, cornering drag, etc., act on a tire of each wheel. Also, the wheel torque changes because of change of the grounding or road-contact load under centrifugal force during turning. Therefore, it is preferable to adjust wheel torque input that is fed back in the vibration damping control through the drive output control as described above, in a manner different from the time when the vehicle travels straight. More specifically, if a steering angle is given to a tire during turning of the vehicle, a cornering drag is generated in a direction opposite to the traveling direction of the vehicle, and the sum of components of the cornering drag and the cornering force in the rotational direction of the wheel (rolling resistance) arises in a direction opposite to the rotational direction. The resulting force is applied in such a direction as to reduce the wheel torque so as to reduce rotation of the wheel. Thus, in JP 2008-105471 A, it has been proposed to increase a control amount for the vibration damping control, so as to make up for reduction of the wheel torque, during turning of the vehicle. On the other hand, if the wheel torque starts changing in the decreasing direction at the start of turning of the vehicle, and the drive torque is increased so as to increase the wheel torque under vibration damping control, in response to the reduction of the wheel torque, the drive torque rapidly increases if the steering angle changes rapidly, and the vehicle turns quickly. As a result, the driver may get a feeling of insecurity. Thus, in JP 2008-105472 A, it has been proposed to stop driving force control based on wheel torque, when the steering angle velocity is larger than a predetermined value.
The motion of a vehicle in a situation where the vehicle turns during execution of vibration damping control for suppressing vibration of the vehicle body by controlling drive output will be described in more detail. Initially, in general, when the driver operates the steering wheel, and the vehicle starts turning, the rolling resistance increases in each wheel (turning resistance), as described above. As a result, nose-down of the vehicle body takes place. Then, the grounding load of the front wheels increase due to the nose-down. As a result, the cornering force increases, and the yaw moment increases, so that the yaw rate effectively appears or increases. However, if the vibration damping control is being performed during steering at the start of turning of the vehicle, the effect of the control functions to eliminate the nose-down, whereby the grounding load of the front wheels is reduced, as compared with the case where the vibration damping control is not performed. As a result, the yaw moment is reduced, and the response of the yaw rate (the rapidness of the increase) is deteriorated. The deterioration of the response of the yaw rate may make the driver feel strange or uncomfortable. Meanwhile, even if the vibration damping control is completely stopped during steering operation, as in JP 2008-105472 A, the driver may feel strange or uncomfortable. This is because the effect of the vibration damping control is suddenly eliminated, or the response of the yaw rate is rapidly changed. In this connection, similar phenomena may also occur when the steering angle is returned to 0 during turning of the vehicle.